Method of electrophoretic deposition of cathodoluminescent materials

ABSTRACT

An electrodeposition process for the cataphoretic deposition of cathodoluminescent materials to produce a film cemented to a substrate surface with the oxide or hydroxide of a soluble metal salt. The film is formed by electrolyzing a suspension of the luminescent material in a quiescent solution of a soluble salt of the corresponding metal in an organic solvent miscible with water containing from about 0.01 to about 1 percent water by volume, a cathodic depolarizing agent, and having an electrolyte concentration in the order of 0.5-25 mg/ml.

llnited States Patent Grosso et al.

Jan. 30, 1973 METHOD OF ELECTROPIIORETIC DEPOSITION OF CATHODOLUMINESCENT MATERIALS Inventors: Patrick F. Grosso, Stamford; Robert E. Rutheriord, Jr., New Canaan, both of Conn.; Donald E. Sargent, Schnectady, N.Y.

Assignee: Columbia Broadcasting System, Inc.

Filed: July 17, 1970 Appl.No.: 55,777 I Related U.S. Application Data Continuation-impart of Ser. No. 792,575, Jan. 21, 1969, Pat. No. 3,551,306.

U.S. Cl ..204/181 Int. Cl ..B0lk 5/02, C23b 13/00 Field of Search ..204/l8l [56] References Cited UNITED STATES PATENTS 2,851,408 9/1958 Cerulli ..204/181 3,554,889 l/l97l Hyman et al. ..204/l8l Primary ExaminerHoward S. Williams Att0rneySpencer E. Olson [57] ABSTRACT An electrodeposition process for the cataphoretic deposition of cathodoluminescent materials to produce a film cemented to a substrate surface with the oxide or hydroxide of-a soluble metal salt. The film is formed by electrolyzing a suspension of the luminescent material in a quiescent solution of a soluble salt of the corresponding metal in an organic solvent miscible with water containing from about 0.0! to about 1 percent water by volume, a cathodic depolarizing agent, and having an electrolyte concentration in the order of 0.5-25 mg/ml.

' 5 Claims, 1 Drawing Figure DE POS/ T ION COND/ T/ONS ANOOE: CARBON C4 THODE. STAINLES'S STEEL CURRENT: lOmfl VOL 746E: VAR/ED TIME. 5 MIN.

ELECTRODE SPAC/NG: 5/8 //V. P-l6 PHOSPHOR CONCENTRATION IN SUSPENSION. lmg/m/ P-l6 PHOSPHOR OEPOS/TEO mg/cm MAGNESIUM N/TRATE (Mg /NO 'EH OI CONCENTRATION mg/m/ PATENTEUJAISO ms INVENTORS. PA TRICK F. GROSSO DONALD E. .SARGE/VT ROBE 7' E. RUTHERFORD, JR.

ATTORNEY METHOD OF ELECTROPHORETIC DEPOSITION OF CATHODOLUMINESCENT MATERIALS This application is a continuation-impart of application Ser. No. 792,575, filed Jan. 21, 1969, now U.S. Pat. No. 3,551,306.

BACKGROUND OF THE INVENTION Thin, relatively uniform layers of luminescentmaterials have been obtained by the practices of the prior art, which include the forming of luminescent coatings by electrophoretic deposition, an example of which is described in U.S. Pat. No. 2,851,408 to Cerulli. Briefly, in the Cerulli process an electrolyte is mixed with a suspension of luminescent materials in an alcohol-water solvent containing from 90 to 99 percent alcohol in order to charge the luminescent particles. The charged particles are electrophoretically deposited on a cathode and the electrolyte leached away. More particularly, in the patented process, the coating suspension is agitated and subjected to the influence of an electric field to bring about deposition of the suspended luminescent material. The main constituent of the suspending medium is 90 to 99 parts by volume of alcohol and 10 to 1 parts water, the patent teaching that using less than 1 percent of water impairsthe solubility and ionization of the electrolyte with resultant impairment of the deposited coating.

The processemploys electrolyte concentrations of 10' to 10 moles/liter in alcohol-water suspensions which, for magnesium nitrate,v for example, equals 0.002 mg/ml to'2.46 mg/ml, it being suggested that if this range is exceeded at the lower end the deposit will either benegligible or at best non-uniform in nature, and that if too much electrolyte is used the deposit is rough and irregular. The electrolyte is deposited along with the luminescent material, and Cerulli having found that the electrolyte poisoned the co-deposited luminescent material particles, removes the electrolyte by soaking the substrate in an alcohol-water solvent.

While the Cerulli process may be satisfactory for the deposition of certain luminescent materials,,attempts to'employ it for the-deposition of phosphors, such as P-16 phosphor used on cathode ray tube screens, have been totally unsatisfactory. For example, when it was attempted to deposit P-l6 phosphor, using the preferred formulation of 95 parts by volume of isopropyl alcohol to parts of water, and thorium nitrate electrolyte in a concentration of moles/liter, no useable phosphor screen was deposited. Thus, the Cerulli process appears to be useful only for the deposition of coarse electroluminescent screens of sulfide phosphors since it demonstrably is unsatisfactory for the deposition of fine grain, high resolution, cathodoluminesc'ent screens of silicate, oxide and sulfide phosphors.

It is, therefore, an object of the present invention to overcome the foregoing shortcomings of the Cerulli process by the provision of an improved method of electrophoretically depositing a thin, uniform, smooth, strongly adherent layer of luminescent material, such as phosphors used in cathode ray tubes, on a conducting surface.

SUMMARY OF THE INVENTION The process according to the present invention is similar to the Cerulli process in that it involves the electrophoretic deposition from a mixture which contains a suspension of phosphor particles in a medium including an organic solvent and water, and an electrolyte. Contrary to the teaching of Cerulli, however, it has been found that a film of an alkaline earth oxide or hydroxide, or other related metal oxides or hydroxides, can be formed on a conducting surface by electrolyzing' an essentially anhydrous solution of a salt of that metal in a water-miscible organic solvent. More particularly, the solution which is electrolyzed, conveniently described as essentially anhydrous, contains water in the range of 0.01 to 1 percent by volume, and a suitable electrolyte at a concentration in the range of 0.002 to 0.025 mg/ml. The phosphor material to be deposited is suspended in the solvent-electrolyte solution in concentrations in the range of about 1 to mg/ml. The film produced by the process is strongly adherent and provides exceptionally strongbonding of the phosphor material to the substrate, and is very smooth and uniform. The codeposited oxide or hydroxide film shows no poisoning effect whatever, and need not be.

removed, and the spectral response and persistence of the resulting phosphor screen are substantially the same as for settled screens.

' Preferably, the organic solvent also includes a cathodic depolarizing agent to prevent or minimize for- 'mation of bubbles of nascent-hydrogen on and in the vicinity of the conductive substrate'which otherwise deleteriously affect the uniformity of the deposited film. Suitable cathodic depolarizers include solvents such as ketones and aldehydes, as well as reducing agents such as various sugars and dyestuffs. A typical solvent system incorporating a cathodic depolarizing agent consists of 74 percent acetone, 25 percent isopropyl alcohol and 1 percent water, all by volume.

DESCRIPTION OF THE DRAWING The significance of the differences of the present process from the Cerulli process, and a better understanding of the invention, will be apparent from the following description and the accompanying drawings, in which:

The FIGURE is a series of deposition curves illustrating the effect of solvent-water concentration, and electrolyte concentration, on the amount of phosphor deposited.

DESCRIPTION OF THE PREFERRED A EMBODIMENT Although in the commercial practice of the process deposition apparatus appropriate to the size and number of screens to be deposited would be used, the process will be described in terms of experimental quantities and laboratory-type equipment.

To determine the effect of electrolyte concentration and of various solvent-water concentrations on the amount, and quality, of phosphor deposited, the following test procedure was employed-The deposition cell consisted of an unstirred vessel of 300 ml capacity, provided with a stainless steel frame supporting a 1 inch diameter test blank in a vertical position. The test blanks, formed of either conductive glassor stainless steel, one of which was used as the cathode in each experiment, were cleaned ultrasonically with acetone and weighed before each experiment. Facing the blank electrode and separated from it by a distance of inch was a row of three it inch carbon rods held in a vertical position which served as the anode. Obviously other suitable inert anode materials may be employed, such as platinum, gold, etc. A variable DC power supply capable of providing potentials of up to 500 volts and currents of up to 250 milliamperes was used to cause deposition.

In a typical operation, 250 mg of P-l6 phosphor powder (calcium magnesium silicate) was added to 250 cc of the fluid-suspending medium in which the electrolyte under test had previously been dissolved. The phosphor was allowed to equilibrate with the liquid medium by stirring for 5 minutes and the resulting suspension was then poured into the test cell. A DC potential sufficient to yield a current of milliamperes was applied across the electrodes and deposition was allowed to proceed for 5 minutes at 25 C without stirring. The test cathode was then withdrawn, dried and weighed. The appearance of the deposited layer was examined and noted.

Employing the foregoing procedure, a series of experiments were performed in which various proportions of magnesium nitrate were dissolved in a solvent system consisting of 99 percent isopropyl alcohol and 1 percent water, by volume. After the magnesium nitrate had been dissolved, 250 mg of calcium magnesium silicate phosphor powder (P-l6 phosphor) were dispersed into 250 cc of the suspending medium. The magnesium nitrate charges the particles of added phosphor, resulting in a stable suspension. The suspension thereupon was placed in the test cell and the phosphor deposited according to the above-described deposition conditions. The resulsts were as shown in Curve 1 of the FIGURE wherein the vertical axis is the weight of phosphor deposited on the test cathode and the horizontal axis is the concentration of the magnesium nitrate (Mg (N09 6H O) electrolyte in mg/ml.

It will be apparent from examination of Curve 1 that optimum deposition occurred with an electrolyte concentration of approximately 8 mg/ml in a suspension containing 1 mg/ml of phosphor particles. The aforementioned concentrations of organic solvent, water and electrolyte resulted in an extremely stable suspension which was useable after hours of standing, without agitation. The latter result is believed attributable to the fact that the specified concentrations and selections of constituents cause the phosphor particles to absorb a sufficient charge to maintain a sufficiently high Zeta potential between particle and liquid to maintain the particles in suspension, but yet not so excessive as to cause electrolysis. Curve I further indicates that if magnesium nitrate electrolyte in excess of about 8 mg/ml is added, optimum charging of the phosphor particles is not attained, and, if more is used, the amount of phosphor deposited also decreases because the increase in conductivity tends to favor electrolysis over electrophoretic deposition.

The use of magnesium and related metallic salts as the electrolyte results in the formation and deposition of a strongly adherent film of magnesium hydroxide which serves as the cementing agent for the simultaneously cataphoretically deposited phosphor. When magnesium salts are electrolyzed in the abovedescribed solutions, hydrogen rather than magnesium is liberated at the cathode. This liberated hydrogen is apparently taken up in the present process by the cathodic polarizing agent (to be discussed below), but regardless of what happens to it, there is an increase in hydroxyl ion concentration (or pH) in the vicinity of the cathode. This results in the formation of the aforementioned adherent form of magnesium hydroxide, the cathode reactions being represented as follows:

That an adherent magnesium hydroxide film is formed under the above-described conditions is indicated by the following test, described in detail in the parent application, Ser. No. 792,575: The addition of phenolphthalein to the system with the phosphor omitted gives a very adherent, bright red film on the cathode when current is passed, which is believed to be a combined film of magnesium hydroxide and the magnesium salt of the phthalein (colored, basic form).

The foregoing experiments were then repeated for different organic solvent-water concentrations to determine its effect, as well as the effect of electrolyte concentration, on the amount of P-16 phosphor deposited under the same controlled conditions. Four additional solvent-water concentrations were tested: (1) 100 percent isopropyl alcohol (i.e., no water); (2) 99.5 percent isopropyl alcohol and 0.5 percent water; (3) 98 percent isopropyl alcohol and 2 percent water, and (4) percent isopropyl alcohol and 10 percent water. In the experiments in which water was not added (Curve 2 in the FIGURE), the maximum deposition (about 20 percent less than for the 99-1 percent concentration) occurred with approximately 22.5 mg/ml of the nitrate, almost three times that required when the 1 percent by volume water system was used. The results of the 0.5 percent water concentration, depicted in Curve 3, are less satisfactory than those achieved with 1 percent water, mainly in that less P-l6 phosphor was deposited per unit time. In this case, optimum deposition occurred with an electrolyte concentration of approximately l0 mg/ml (as contrasted with 8 mg/ml with the 1 percent water system), producing a smooth, uniform, strongly adherent film. When solvent-water concentrations of 98 percent alcohol 2 percent water and 90 percent isopropyl alcohol 10 percent water were used, very little phosphor was deposited at any electrolyte concentration, and the small amount that was deposited was rough and not acceptably adherent. The deposition results are illustrated in Curves 4 and 5, respectively, of the FIGURE. This series of experiments demonstrate that the organic solvent-water concentration, which, in turn, determines the amount of electrolyte required for satisfactory deposition, is a critical feature of the process.

Even at the water concentrations of 0.5 to 1 percent by volume, bubbling at the cathode caused by the evolution of gaseous hydrogen, was observed; such bubbling deleteriously affects the rate of deposition and quality of the deposited phosphor film. An important aspect of the present process which essentially eliminates this bubbling is the use of a cathodic depolarizing agent in the organic solvent-water mixture. More specifically, instead of only alcohol, an organic solvent consisting of approximately 75 percent acetone and 25 percent alcohol, was used with water concentrations in the range of 0.5 to 1 percent by volume. The acetone prevents evolution of gaseous hydrogen, presumably by acting as an acceptor of any hydrogen generated, according to the formula:

(acetone) (isopropyl alcohol) When no hydrogen evolves, very smooth, compact, adherent phosphor screens are deposited. On the other hand, when hydrogen is present in the vicinity of the cathode, the deposited phosphor is non-uniform, loosely .packed, low in weight, and most significantly, nonadherent, even to the point that the film slides off a metal electrode during removal from the electrolytic bath.

Phosphor screens deposited in accordance with the present inventiondo not require removal of ionizable salts deposited with the phosphor particles as required by the Cerulli process. Very smooth, adherent, high resolution P-16 phosphor screens have been deposited on a metal anode, such as stainless steel, and the screen incorporated in cathode ray tubes. Thev luminescent characteristics of the phosphor, even though the magnesium ionizable salt'was not removed after deposition, did not change with age or use. In fact, the initial radiant output was much higher than conventional high resolution P-l6 screens of the same phosphor density prepared by the settling process, the improved performance being achieved because the electrophoretically deposited screen does not have the silicate bonding agent of conventional tubes which tends to decrease the radiant output of the phosphor. During the operating life of the screen, the radiant output of the electrophoretically deposited screen decreases at almost the same rate as settled phosphor screens, and show no poisioning effect whatever due to the included ionizable salt. Also, the spectral. response and persistence of the electrophoretically deposited screen are substantially the same as for settled screens.

' Although the present process has been described in connection with the deposition of P-16 phosphor, tests have shown that it can be successfully used for the cataphoretic deposition of any phosphor which can be positively charged for deposition at a cathode. These materials include, but are not limited to, other silicates such as zinc silicate, calcium silicate, magnesium silicate; phosphates such as zinc phosphates; sulfides such as zinc sulfide and zinc cadmium sulfide; fluorides such as zinc magnesium fluoride; chlorides such as potassium fluoride; tungstates such as calcium tungstate; and oxides such as zincoxide. It has beenfound that concentrations of phsophor'in the order of 0.01 to 0.1 gms/ml, or '1 to 100 mg/ml, is preferable, mechanical agitation being necessary to maintain the phosphor in suspension if higher concentrations are used. Since in the present process a quiescent solution is electrolyzed, the concentration of the phosphor suspensions is less than in the Cerulli process, for example, in which the suspension is agitated. Phosphor particle sizes in the range of 2 to 10 microns are preferred, particle sizes at the lower end of this range being ideal for depositing a smooth phosphor layer. Particle sizes smaller than 2 microns, however, tend to have poor luminescence.

Other salts than magnesium nitrate from which electrodeposited films can be formed by the present process include magnesium chloride, aluminum nitrate, calcium nitrate, strontium nitrate, zinc nitrate, zinc chloride, copper nitrate, copper chloride, gallium nitrate, cobalt chloride, silver nitrate, copper sulfate, gold chloride, maganese chloride, barium nitrate, barium chloride, barium acetate, lithium bromide, lithium nitrate, cesium nitrate, and aluminum chloride. It is preferable to use non-reducible salts to avoid electroplating of the metal during the deposition of the metal oxide or hydroxide. When reducible salts such as thorium nitrate, and nitrates, chlorides or bromides of zinc, nickel, lead, copper, iron (ferric), cobalt and maganese are used they tend to reduce to the metal and give either poor or very discolored deposits (probably containing respective metals or lower valence compounds) and/or they are poorly adherent.

It is to be noted, however, that while a wide range of metal oxides or hydroxides may be formed when the present invention is used in the electrophoretic deposition of phosphor films, other considerations must also be taken into account in selecting suitable electrolytes. The preferred anions for the electrolyte are the nitrates, chlorides and, to a lesser extent, the bromides. Nitrates and chlorides of magnesium, aluminum and lithium give good depositions; the sulfates of these and other metals perform relatively poorly in the electrodeposition of phosphors. 1

In the formation of a dispersion of the phosphor material to be electrophoretically deposited, the individual phosphor particles must be given a positive charge so that they will migrate to the negative cathode. Salts of metals such as magnesium are effective to charge many phosphor materials because the metal ion is adsorbed onto the surface of the phosphor giving it a positive charge surrounded by a negatively charged ionic double-layer of the anion of the absorbed metal ion.

It will be recognized that the objects of the invention have been achieved by providing an improved method for electrophoretically depositing thin, uniform layers of phosphor, having excellent luminescent characteristics, on a conducting surface. The success of the process is largely due to the use of an essentially anhydrous solvent and the use of a cathodic depolarizing agent for minimizing gas bubbling at the cathode.

I claim:

l. The method of forming a thin uniform layer of cathodoluminescent particles on an electrically conductive surface comprising, forming an organic solvent-water electrolyte suspending medium consisting of about 99.99 to 99 percent of an organic solvent containing a cathodic depolarizing agent which is miscible with water and 0.01 to 1 percent water, and from 0.05 to about 25 mg/ml of a salt selected from the group consisting of the chlorides, bromides, acetates and nitrates of calcium, magnesium, aluminum, zinc, copper, cobalt, gallium, manganese, barium, lithium, cesium, silver and gold, forming a suspension of a finely divided cathodoluminescent material ranging in particle size distribution from sub-micron size to 10 microns in said medium in the proportions of 1 to grams of cathodoluminescent material per lOO ml of said suspending medium, placing an anode and a cathode on which said layer is to be formed in said suspension, and applying a potential difference of from about 10 to about 500 volts between said anode and said cathode to yield a current density at said cathode of from about 1.0 to about 50 milliamperes/cm for a sufficient time to co-deposit an adhering film of a metal oxide or hydroxide of said salt and cathodoluminescent material of the desired thickness on said cathode.

2. The method according to claim 1 wherein said cathodic depolarizing agent is acetone.

3. The method according to claim 1 wherein said or- 

1. The method of forming a thin uniform layer of cathodoluminescent particles on an electrically conductive surface comprising, forming an organic solvent-water electrolyte suspending medium consisting of about 99.99 to 99 percent of an organic solvent containing a cathodic depolarizing agent which is miscible with water and 0.01 to 1 percent water, and from 0.05 to about 25 mg/ml of a salt selected from the group consisting of the chlorides, bromides, acetates and nitrates of calcium, magnesium, aluminum, zinc, copper, cobalt, gallium, manganese, barium, lithium, cesium, silver and gold, forming a suspension of a finely divided cathodoluminescent material ranging in particle size distribution from sub-micron size to 10 microns in said medium in the proportions of 1 to 10 grams of cathodoluminescent material per 100 ml of said suspending medium, placing an anode and a cathode on which said layer is to be formed in said suspension, and applying a potential difference of from about 10 to about 500 volts between said anode and said cathode to yield a current density at said cathode of from about 1.0 to about 50 milliamperes/cm2 for a sufficient time to co-deposit an adhering film of a metal oxide or hydroxide of said salt and cathodoluminescent material of the desired thickness on said cathode.
 2. The method according to claim 1 wherein said cathodic depolarizing agent is acetone.
 3. The method according to claim 1 wherein said organic solvent-water solution consists essentially of from about 0.01 to 1 percent by volume water, about 25 percent by volume isopropanol, with the balance thereof being acetone.
 4. The method according to claim 1 wherein said organic solvent-water solution consists essentially of 1 percent by volume water, about 25 percent by volume isopropanol, with the balance thereof being acetone, and wherein said salt is magnesium nitrate. 